Additional information about the condition of the

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Additional information about the condition of the rotor windings can be found by comparison of the transient behavior of symmetrical winding branches, each connected to a slip ring. INSULATION RESISTANCE OF THE ROTOR WINDINGS AND DETERMINATION OF THE POLARIZATION INDEX To determine the polarization index and insulation resistance of rotor windings a dedicated test can be performed. The insulation resistance of the rotor windings is measured by means of a digital Megger. During the measurement a DC-voltage of approximately 500 V is applied. The measurements are performed as a function of the time after application of the voltage. The measurements are performed on the rotor winding with the other machine winding earthed, together with the frame of the stator. It is important to register the temperature of the winding for the evaluation of the result. The leakage current that occurs when the winding insulation is subjected to a DC-voltage is a measure for the condition of its insulation. A distinction can be made here between the time- dependent effects during the charging of the winding capacitance and the non-time-dependent effects which occur as a result of moisture and/or tracking inside or on the surface of the winding insulation. The leakage current is measured at a specified DC- voltage level by means of an insulation resistance meter (Megger). From the measured insulation resistance as a function of the time, the polarization index is determined. The ratio of the insulation resistance values measured 10 minutes and 1 minute after the application of the DC-voltage, is called the polarization index. The polarization index and the insulation time constant are measures for the evaluation of the insulation condition with respect to cleanliness and the presence of moisture. The temperature of the winding is of importance. ENERGY 15 The admittance is presented in curves for the circuit inner slip ring to shaft (curves inner long and inner short) and for the circuit outer slip ring to shaft (curves outer long and outer short). The expression short and long relates to the length of the applied interconnecting cable wires between pulse generator and rotor. The length of the cable wires is of minor importance on the measurement. Furthermore, for frequencies up to 20 kHz, the admittance of both circuits is identical. Non-destructive testing & plant diagnostics
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ENERGY 16 Failure of a generator stator conductor due to electrical or mechanical problems will lead to long forced outage of the unit resulting into huge revenue loss. Therefore it is necessary for judgment of insulation deterioration to carry out a diagnostic test of the stator coil. To evaluate the residual life from experience and the accumulated data, DNV KEMA offers a full set of diagnostics to assess the generator stator condition, like: Insulation resistance Polarization index Capacitance and dielectric losses Partial discharge Voltage tests MEASUREMENT OF THE DC-RESISTANCE OF THE STATOR WINDINGS In order to detect presence of defects in the stator windings
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  • Fall '19
  • Electrical generator, Stator, DNV KEMA

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